The present invention relates to powders and products of tantalum, niobium, and their alloys having low oxygen contents, and processes for producing the same.
One common method of producing powder metal products of tantalum, niobium or alloys of such metals with each other, and either or both with other metals, is to first cold isostatically press the powder into a preform, such as a bar or rod. The preform is resistance sintered at a relatively high temperature to produce a formed product of tantalum, niobium or their alloys. Generally, for resistance sintering, the ends of the preform are clamped between water-cooled copper terminals in a high vacuum chamber and then the preform is heated by passing an electrical current through the preform. The resistance sintering simultaneously lowers the oxygen content and densifies the preform.
However, there are many disadvantages in utilizing resistance sintering to densify and remove oxygen. First, resistance sintering may only be utilized to produce products of certain limited shapes, generally bars or rods. For resistance sintering, the cross-section of the preform must be uniform along the path of electrical current in order to prevent localized overheating and hot shorting. Additionally, the cross section must be small enough so that the oxygen reduction in the center of the preform occurs before the disappearance of the interconnected porosity. For effective oxygen removal, preforms greater than about 1.5 inches in their shortest dimension are not resistance-sintered. Still further the preform must be small enough to prevent sagging associated with creep and hot pressing during unsupported resistance sintering. Thus, the preforms generally do not weigh greater than about 35 lbs.
The present invention relates to tantalum and/or niobium powder and more particularly to tantalum powders usable in making powder metallurgy components that can be further processed into rod, wire, sheet, foil and other mill or fabricated parts or simply made in net-shape by conventional powder consolidation methods followed by surface finishing and/or small dimensional modifications. Such products are also usable as fully dense coatings to modify the surface chemistry of other mill products or fabricated parts.
It is well known to make tantalum powders for use as sintered anodes for electrolytic capacitors, by hydriding an ingot or chips from an ingot of tantalum, comminuting (taking advantage of the embrittlement that results from this massive hydriding) to powder form and then dehydriding to form tantalum powder. Such powders can be used to produce capacitors with low electrical leakage. In principle such process is also applicable to niobium but it is not very practical.
It is also known to deoxidize tantalum or niobium capacitor powders (however made) in primary or secondary (agglomerated) forms by contact with vapors of alkaline earn, metals to effectively getter oxygen at the powder surface and remove it as an alkaline earth, metal oxide by acid leaching and/or volatilization.